1. Field
The present invention relates generally to position determination, and more specifically to a method and apparatus for identifying transmitters in a wireless communication system using power predictions.
2. Background
It is often desirable, and sometimes necessary, to know the position of a wireless user. For example, the Federal Communications Commission (FCC) has adopted a report and order for an enhanced 911 (E-911) wireless service that requires the location of a wireless terminal (e.g., a cellular phone) to be provided to a Public Safety Answering Point (PSAP) each time a 911 call is made from the terminal. In addition to the FCC mandate, service providers may use location services (i.e., services that identify the position of wireless terminals) in various applications to provide value-added features and possibly generate additional revenues.
The position of a wireless device may be determined based on various position determination systems. One such system is the well-known Global Positioning System (GPS), which is a “constellation” of 24 well-spaced satellites that orbit the earth. Another such system is a wireless communication system, e.g., a cellular communication system that includes a number of base stations to support communication with a number of user terminals.
In general, an accurate position estimate for a wireless device may be obtained based on (1) distances or ranges from the device to a sufficient number of transmitters (typically three or four), and (2) the locations of these transmitters. Each transmitter may correspond to a GPS satellite or an earth-bound base station in the cellular system. The range to each transmitter may be estimated based on the signal transmitted by the transmitter. The location of each transmitter can typically be ascertained if its identity is known. The identity of each transmitter may be provided in the signal transmitted by the transmitter.
In many instances, the number of signals required to compute an accurate position estimate cannot be received by the wireless device, or the range information is not available. In these instances, a coarse position estimate for the wireless device may be obtained based on other information for the transmitters whose signals are received by the wireless device. For example, a coarse position estimate may be obtained for the wireless device based on knowledge of the locations and/or the coverage areas of the base stations received by the device. In any case, the identity of a base station would need to be ascertained before its signal can be used for position determination.
For a CDMA communication system, each base station may be identified based on various types of information. First, each base station may be unambiguously identified based on information included in certain overhead messages transmitted by the base station. However, to receive and recover these messages, the wireless device would need to establish communication with the base station. Second, each base station may be identified based on a pseudo-random number (PN) sequence assigned to the base station. For a CDMA system, each base station is assigned a specific offset of the PN sequence, which is different from the offsets assigned to neighboring base stations. Each base station uses its assigned PN sequence to spectrally spread data prior to transmission over the air. However, due to a limited number of PN offsets available, multiple base stations in the system may be assigned with the same PN offset. Thus, it may not be possible to unambiguously identify a given base station based solely on the PN phase of the signal received from that base station.
There is therefore a need in the art for a method and apparatus that can identify the transmitters in a wireless communication system.